1. Field of the Invention
The present invention relates to a process for forming shaped ceramic articles of calcium phosphate such as hydroxylapatite and calcium tertiary phosphate or a mixture thereof.
2. Description of the Prior Art
According to the conventional processes, shaped ceramic articles of calcium phosphate are produced by press molding calcium phosphate powder and sintering the preform under normal pressure (dry process); by hot-pressing calcium phosphate (dry process); or by drying and sintering, without crushing, the filter cake which is obtained when the precipitates of synthetic apatite are dehydrated by vacuum pressure filtration or centrifugal dehydration (wet process). An example of the wet process is disclosed in U.S. Pat. No. 4,097,935.
The dry process for calcium phosphate molding has a disadvantage in that calcium phosphate sticks to the mold face, making it difficult to form shaped articles of uniform structure. In addition, the dry process is not suitable for producing small articles of complex shape. They are usually produced by fabricating a previously made large block and then cutting the large block to an article of desired shape by tools. As compared with the dry process, the wet process is easier to perform, and it is suitable for producing small shaped articles. Yet, it has a disadvantage in that the filter cake is liable to cracking or fracture in the drying process. Moreover, the wet process is only capable of producing shaped articles in the form of thin plate (10 to 11 mm in length, 4 to 5 mm in width, and 2 to 3 mm in thickness), as described in U.S. Pat. No. 4,097,935.
The improvement on the wet process is described in U.S. Pat. No. 4,312,821. According to this U.S. Patent, an aqueous suspension of calcium phosphate is cast into a mold, with the surface thereof coated with a lubricant, followed by drying under a mild condition. This process permits the production of larger articles (measuring 18.times.9.times.9 mm). This process still has a disadvantage in that the castable aqueous suspension of calcium phosphate contains as much water as 75 to 95% and the high water content causes minute cracks even when drying is performed under a mild condition. This makes it difficult to produce crack-free products in high yields and leads to troublesome drying.